It is not uncommon to see two fast bowlers hard at work in the nets, competing for one spot. At the WACA on Wednesday, Australia took the idea to the extreme. Mitchell Johnson, Mitchell Starc, John Hastings and Josh Hazlewood were all running in and doing their darnedest to impress the selectors ahead of Friday's third Test. None of them knew how many positions would be up for grabs. It could be one, it could be four. It could be anything in between.

The first deciding factor is how Peter Siddle and Ben Hilfenhaus handle their training runs on Thursday, the end of a short turn-around following their heavy workloads in the Adelaide Test. The other issue is whether Nathan Lyon will be required, or if the selectors will choose a four-man pace attack, as they did with great success against India last summer. Lyon would enjoy the drift provided by the Fremantle doctor, the sea breeze that blows in the mid-afternoon in Perth, but he's not the only one.

Johnson has had success at the WACA, notably against the South Africans in 2008-09, when he took 11 wickets and was unplayable late on the second afternoon. But his fellow left-arm seamer Starc could justifiably expect to have the inside running, having been 12th man in the first two Tests. He used the cross-ground breeze to his advantage in Perth last season, when it helped him curl the ball in towards India's right-hand batsmen, and one delivery that swung in and trapped Sachin Tendulkar lbw was especially memorable.

"I have fond memories from last year," Starc said. "It's a nice place to come and bowl, a bit of extra pace and bounce than you get at the SCG where I play. There's more in it for the bowlers but last year it was a pretty good cricket wicket. Hopefully if I get the chance to come out here again I can bowl similar to what I did last time."

Certainly, one of Starc or Johnson should play. Even if the only position available is as a replacement for the injured James Pattinson, the left-arm variety would ask different questions of South Africa's batsmen. Graeme Smith, in particular, has been troubled by left-arm seamers in recent years. Not that Hastings and Hazlewood are out of contention.

Should an all-pace attack be employed - a distinct possibility given the success the Australians had against India, and the likelihood that the pitch will need to spend time under cover with rain forecast on match eve and the first day of the Test - Hastings and Hazlewood might be battling each other for one spot. Both are tall men who extract impressive bounce, but do not deliver the ball at express speed. Neither has played Test cricket before.

Hazlewood, 21, has been viewed as a national prospect since he first played for New South Wales at the age of 17. Nothing in his first-class form this summer - nine Sheffield Shield wickets at 46.33 - suggests he is quite ready for Test cricket, but the selectors have a hunch about him. By his own admission, his height is a key weapon, along with his consistency, but at 196 cm he has no discernible advantage over Hastings, who stands 195cm.

Hastings, 27, has the form that Hazlewood lacks. This summer, he has collected 22 Shield wickets at 19.09. Having missed all of last season due to a shoulder reconstruction, Hastings spent countless hours in the gym and has returned better for it. Compared to state team-mates Pattinson and Siddle, Hastings can appear innocuous, but he has lifted his pace this season and has reaped the rewards.

"When I wasn't bowling or batting I spent a lot of time on my fitness, working on my strength and conditioning in the gym with David Bailey, the now Australian strength and conditioning coach," Hastings told ESPNcricinfo last month of his time on the sidelines. "We worked our butts off to try to get me back. I think a yard of pace has probably helped me get a few more wickets than I normally would have in four-day cricket, and a little more durability as well, so I can continually back up my spells."

Although it is difficult to see Hastings and Hazlewood both playing, there is the outside chance that a full swap of Australia's attack could take place, should Siddle and Hilfenhaus struggle at training on Thursday. One certainty is that Australia's batsmen can expect a torrid time in the nets on match eve.

"The guys in and around the squad know there's a few places that might be up for grabs so they're certainly having a good crack in the nets," Shane Watson said on Wednesday. "That's good for the batsmen because we certainly get a competitive, high quality net session, but it's also great to be able to see how they're travelling as well."

Brydon Coverdale is an assistant editor at ESPNcricinfo. He tweets here